Numerous configurations of diagnostic devices incorporating planar electrodes have been described in the prior art. Planar electrodes contained within these prior art devices have been used as sensors (for example U.S. Pat. Nos. 4,053,381, 4,133,735, 4,225,410), in electrokinetic devices either as elements of electro-osmotic pumps or as electrodes for electrophoretic separation (for example U.S. Pat. No. 4,908,112), for electrical stimulation (U.S. Pat. No. 5,824,033), and the like. Common to all such prior-art devices is a planar electrical conductor with one location at which a contact is made to an external circuit or a measuring device, and a second location, the electrode, at which contact is made to an electrolyte. Often the electrode consists of one or more additional layers between the conductor and the electrolyte.
A design objective common to all such devices is the r&quirement to electrically insulate the conductor including the contact made to an external circuit from the electrode region in proximity to the electrolyte. Two general configurations have addressed this in the prior art. 1) Devices with planar conductors on planar supporting substrates have been configured so that the electrodes are contacted on the same surface as the electrolyte contact. Devices of this art are typically elongated so that there is a spatial separation between electrical contacts and electrolyte and there is electrical isolation of the conductor and the region of contact to the external circuit by an insulating barrier interposed between these two locations on the same electrode surface. 2) Devices with planar electrodes on planar insulating support substrates configured so that contact to the external circuit is made on the opposite surface to that at which the electrolyte makes contact. The electrode often traverses the substrate so that it is interposed between the electrolyte on one surface and the conductor contacts on the other surface. Devices of this type also are often elongated to spatially separate the contacts from the electrolyte.
Prior art planar diagnostic devices containing electrodes with same-surface contact configuration have been manufactured by a variety of different technologies. U.S. Pat. Nos. 4,133,735, 4,591,793, 5,727,548 for example disclose devices with electrodes made by thick film fabrication processes (printed circuit board technology, screen-printing, dispensing and the like). U.S. Pat. Nos. 4,062,750 4,933,048 and 5,063,081 disclose chip-like devices containing electrodes made by thin film micro-fabrication processes on silicon substrates. U.S. Pat. Nos. 4,053,381 and 4,250,010 disclose planar devices fabricated on conductor slabs or foils.
Prior art planar diagnostic devices containing electrodes with opposite-surface contact designs have been manufactured by a variety of technologies. U.S. Pat. No. 4,549,951 for example discloses thick film devices. U.S. Pat. Nos. 4,225,410 and 4,874,500 for example disclose micro-fabricated thick film devices.
Numerous electrode configurations have been disclosed in which the electrode forms part of a probe to be immersed in, or otherwise directly contacting a fluid. For example the prior art features planar electrode devices on catheter probes (for example U.S. Pat. No. 4,449,011), flexible electrode structures for subcutaneous measurement (for example U.S. Pat. No. 5,391,250) or for electrical stimulation (U.S. Pat. No. 5,824,033), as well as planar electrodes in diagnostic strip configuration for application of blood drops (for example U.S. Pat. Nos. 4,591,793 and 5,727,548). Other electrode configurations have been disclosed in which the electrode is designed as an element within a fluidic housing, which housing incorporates channels to provide for a flow of electrolyte to the electrode as well as to perform other fluidic manipulations such as calibration and reagent additions (for example U.S. Pat. Nos. 4,123,701 4,301,414; 5,096,669; 5,141,868; 5,759,364 and 5,916,425).
Mostly, prior art electrodes are expensive to manufacture on a per unit basis because they are structurally complicated or made with expensive materials. Even structurally simple electrodes can be costly at low manufactured volume, if they require specialized tooling and equipment for their manufacture. Such devices can only become inexpensive on a per unit basis when the volume is sufficiently large so that the large fixed cost of tooling and specialized manufacturing equipment can be absorbed by the large volume being produced. The cost issue becomes critical when the electrode is a component of a single-use disposable device. The configuration of devices as unit-use disposables is particularly attractive to users of diagnostic or separation apparatus because the equipment can be very simple and the devices easy to use.
Disposable sensor electrodes of the prior art for home use glucose measurement can exhibit unit costs of only a small fraction of a dollar when manufactured at very large volumes, for example greater than 200 million devices per manufacturer per year. However, there are also numerous diagnostic applications of prior art devices where the unit volume is less than 10 million per year and the cost of manufacturing is on the order of dollars per device. These higher manufacturing costs mean that these prior art devices can be cost-prohibitive for commercially viable lower volume applications.
In the prior art electrokinetic devices, such as for example electrophoretic separation devices, there are no examples of articles of commerce known in which the separation cassette including the transport medium and electrodes is configured as a disposable. In the case of slab gel separation apparatus of the prior art, the slab of gel is used for a single separation then disposed of. The gel slab is cast into a cassette including reusable glass plates forming the upper and lower gel-slab surface and reusable spacers defining separation lanes. There are reusable electrodes for applying the electric field across the transport medium. Such a device is described in U.S. Pat. No. 5,192,412. Further known are micro-scale electrokinetic devices, so called lab-on-a-chip devices, consisting of empty capillary channels formed into planar substrates with integral electrodes, see for example U.S. Pat. No. 4,908,112. Such devices are complex to manufacture and also have not been configured as low cost disposables.
Thus there is a need for disposable sensor electrodes or electrokinetic transport electrodes that can be manufactured at very low cost even at modest manufactured volumes.
To reduce the cost of information storage devices for personal use, a technology unrelated to disposable diagnostic devices, integrated circuit chips are packed into the devices using smart-card technology or IC card technology. See for example the paper entitled “Smart Cards from a Manufacturing Point of View” by Baker in Solid State Technology 1992, 35(10), p 65-70. In the manufacturing of smart cards, integrated circuit chips are assembled, glued and wire-bonded onto chip-carrier modules (for example U.S. Pat. No. 5,041,395). The chip-carrier module includes an insulating layer and a metal layer applied thereto forming a contacting region divided into a number of electrically separate contact areas. Multiple chip-carrier modules are manufactured in a tape format with the insulating layer being in the form of a tape and the metal layer being divided into a plurality of contacting regions along the tape. The purpose of the chip-carrier module is to provide a substrate on which to place and hermetically seal the tiny integrated circuit chip. The purpose is also to provide for contacting means so that the electrical signals can be directed from the chip to metal leads on the chip-carrier module via wire bonds. Because of the requirement for very low cost of the final smart-card device, the chip-carrier module was designed with low cost materials. Moreover, the tape format of the fabricated chip-carrier modules resulted in highly automated reel-to-reel chip assembly, wire-bond and hermetic sealing processes that are also low cost. Although it would be possible to replace the integrated circuit chips mounted on such chip-carrier modules with a conventional lab-on-a-chip device, the resulting manufacturing cost would still be prohibitive for use of this structure in disposable diagnostic devices.